This invention relates to an improved encoder disc for an encoder of the type having a scanning unit for scanning the disc to measure an angular position characteristic of the disc.
Encoders such as shaft angle encoders have been used for some time to provide an electronic signal indicative of the angular position of the shaft to which the encoder is mounted. Such encoders include discs having either absolute or incremental tracks, or a combination of the two. Absolute tracks provide a parameter that varies in accordance with the absolute position of the disc, while incremental tracks provide repetitive signals that can be counted to determine movement away from a reference position.
European Patent Application No. EP 0 276 402 discloses an encoder disc which, as shown in FIG. 2, includes both incremental and absolute tracks. Note in particular the outermost track which varies in width in a linear manner between a minimum width at 0 degrees and a maximum width at 180 degrees. This width variation is indicated in FIG. 4, where the signal U1 is shown as triangular in shape.
Though the triangular waveform produced by the encoder disc of the above-identified No. EP 0 276 402 is suitable for some applications, it is often preferable to provide a measuring track which varies in width sinusoidally rather than linearly. Such sinusoidal waveforms eliminate the cusps of triangular waveforms and associated scanning difficulties. Additionally, processing systems for sinusoidal signals are commonly available.
It is an object of the present invention to provide an improved encoder disc for an encoder of the type described above which provides such a sinusoidally varying measuring track in a particularly simple and cost effective manner.